Murine retrovirus-based packaging cell systems have been shown to spontaneously and sporadically generate infectious murine retroviruses (RCRs). Such viruses possess a broad host range and can infect a wide variety of primate cells including human. Thus, rigorous and sensitive testing is currently required by the FDA for manufacturers to evaluate potential contamination of gene therapy products for murine retroviruses. We initiated studies to evaluate the potential risk of RCRs in primates. Rhesus monkeys were inoculated with an RCR preparation and blood and tissue samples were taken regularly to monitor for retroviral infection. Our results indicated that normal, juvenile monkeys could be infected with RCR. Although infectious virus could only be isolated early post-inoculation, retroviral sequences have persisted over 1 year . We are currently investigating host and viral factors which may activate the latent sequences and its consequences on the host monkey. Retroviral-based gene therapy is an effective delivery system for a therapeutic gene. However, replication-competent retroviruses (RCRs) have been shown to arise spontaneously and sporadically from such packaging cell systems. Due to the infectivity of RCRs in primate cells, including human cells, there is an increased safety concern regarding retroviral-based gene therapy products. Thus the FDA has required rigorous and highly sensitive testing of these products. To support these extensive and fairly expensive requirements, we undertook to evaluate potential risk of RCRs in primates using a monkey model system. Our results have thus far indicated that even normal monkeys can be infected with a relatively low amount of RCR thus confirming the FDA position on the testing requirements. Current work is directed towards long-term effects of potential infection of RCR in primate cells. These studies have also led to development of sensitive assays for RCR testing and guidelines for patient monitoring.